Several years ago, I visited the British Museum and took a pair of photographs of the Lycurgus Cup. The photographs are below:
|The Lycurgus Cup.|
|The same artifact!|
The two photographs depict the same object, but clearly the glass is of a different color in each photograph! This is due to special properties of the glass which scatters and reflects light differently. When light passes through the glass, the blue end of the spectrum is scattered to a greater degree, resulting in its normal red appearance. However, when light is reflected off the glass, the spectrum is reflected about equally, showing a green color.
Of course, the Lycurgus Cup is kept secure behind a wall of glass, which makes tampering with the lighting to create the color changes difficult. I managed to capture the color changes using a camera flash; the reflected light off the flash makes the Cup appear green. Signs of the flash can be seen in the first image above.